The present invention relates to a power transmission mechanism for connecting a drive source to a driven apparatus, which includes a rotating unit. More particularly, the present invention pertains to a power transmission mechanism used in a compressor of a vehicle air conditioner.
A compressor used in a vehicle air conditioner includes a drive shaft and an inner compression mechanism. The compression mechanism is actuated by rotation of the drive shaft. The compressor also includes an electromagnetic clutch to transmit power from the vehicle""s engine to the drive shaft. The clutch is engaged and disengaged based, for example, on the cooling load in an external refrigerant circuit. A typical electromagnetic clutch includes a pulley, an armature and a coupling member. The coupling member connects the armature to an inner hub, which is located at the distal end of the drive shaft. The armature is selectively engaged with and disengaged from the pulley. When the clutch is electromagnetically engaged, engine power is transmitted to the drive shaft by a belt, the pulley, the armature, the coupling member and the inner hub. The coupling member, which is supported by the inner hub, separates the armature from the pulley when the electromagnetic force of the clutch is stopped. Rubber dampener (rubber hub) type and a leaf spring type coupling members are known in the art. The leaf springs used in leaf spring type coupling members are practically ineffective as dampeners.
If a coupling member for coupling an armature with an inner hub is made of rubber, torque fluctuation in the compressor is absorbed by the rubber, which serves as a dampener. However, the coupling member must function not only as a dampener but also as a torque transmitting member. Thus, the spring constant of the rubber must be relatively high to make the coupling member durable. The resonance frequency is determined by the moment of inertia of rotation system of the compressor and the spring constant of the rubber. When the spring constant of the rubber is high, the resonance frequency tends to be higher than the lowest frequency of torque fluctuation generated in the rotation system of the compressor. A typical compressor operates at 1000 rpm to 2000 rpm. If the frequency of torque fluctuation of the compressor substantially matches the resonance frequency when the compressor is operating in a normal speed range, resonance occurs and increases the torque fluctuation. The increased torque fluctuation produces noise in the vehicle.
Using leaf springs, in comparison to rubber, to couple the armature and the inner hub increases the resonance frequency. Further, the leaf springs are ineffective as dampeners. Thus, when resonance occurs, torque fluctuation is excessive, which results in seizing and wearing of contact surfaces of the electromagnetic clutch.
An objective of the present invention is to provide a power transmission mechanism that suppresses vibration and noise due to torque fluctuation and prevents the inner parts of the compressor from being damaged. Another objective is to provide a power transmission mechanism that occupies minimal space.
(1) The present invention relates to a power transmission mechanism that couples a drive source with a rotating unit of a driven apparatus. The power transmission mechanism includes a first rotating body, a second rotating body and a spiral spring. The first rotating body is provided in the drive source. The second rotating body is provided in the driven apparatus and is coupled to the rotating unit. The spiral spring couples the first rotating body with the second rotating body.
The first rotating body, the spiral spring and the second rotating body not only form a power transmission system from the drive source to the driven apparatus, but also a vibrating system, which includes the drive source and the driven apparatus. When power from the first rotating body, which is coupled to the drive source, is transmitted to the second rotating body, which is coupled to the driven apparatus, by the spiral spring, the rotating unit of the driven apparatus receives a load. The load generates repulsion load torque. Therefore, to drive the rotating unit of the driven apparatus, the torque of the drive source must be transmitted to the driven apparatus against the load of the rotating unit. Depending on the type of the driven apparatus, the load and the repulsion load torque fluctuate periodically. Also, depending on the type of the drive source, the transmitted torque periodically fluctuates.
However, according to the present invention, the first rotating body and the second rotating body are coupled to each other by the spiral spring. Therefore, by setting the spring constant of the spiral spring far lower than that of the prior art elastic coupling member, the resonance frequency of the vibrating system can be set outside of the frequency range of torque fluctuation generated in the driven apparatus or in the drive source. Specifically, the spring constant of the spiral spring is determined such that the resonance frequency (fR), which is determined based chiefly on the spring constant of the spiral spring and the sum of the moment of inertia of the rotating unit of the driven apparatus and the moment of inertia of the second rotating body, is lower than the lowest frequency (f1) of torque fluctuation generated in the driven apparatus and/or than the lowest frequency (f2) of torque fluctuation generated in the drive source. Therefore, for any torque fluctuation in the driven apparatus and/or the drive source, the amplitude of the torque fluctuation is not increased by resonance (resonance phenomena) due to the mechanical characteristics of the power transmission system. As a result, excessive noise and damage in the rotation system due to torque fluctuations of the driven apparatus and/or the drive source are prevented.
Further, as described later, the spiral spring occupies relatively little axial space when installed. Thus, using the spiral spring to connect the first rotating body with the second rotating body makes efficient use of a space. Also, the space for the power transmission mechanism is minimized.
(2) In the power transmission mechanism according to the present invention, the spiral spring has at least one arm that spirally extends from the center to the periphery. The outer end of the arm is preferably coupled to or engaged with the first rotating body, and the center of the spiral spring, or the inner end of the arm, is preferably attached to the second rotating body. This defines a preferably shape or structure of the spiral spring. One of the structural characteristics of the spiral spring in the present invention is the shape of the arm, and the number of the arms is not important. However, when the spiral spring has a plurality of arms, the arms are preferably spaced apart by equal angular intervals about the center of the spiral spring. Equally spaced apart arms effectively stabilize the orientation of the spiral spring when the spring is rotating to transmit power.
(3) In the power transmission mechanism according to the present invention, each arm of the spiral spring extends for at least a half turn about the axis of the spiral spring. The outer end of the arm, which is coupled to or engaged with the first rotating body, preferably includes a thick portion located at the opposite side of the center axis of the spiral spring from the corresponding outer end. The cross-sectional area of the thick portions of the arms is preferably larger than the cross-sectional area of the remainder of the arms.
If the length of each arm of the spiral spring is at least long enough to corresponds to a half turn about the center of the spring, the part of each arm that is located on the opposite side of the center axis of the spiral spring from the corresponding outer end requires the greatest strength. Therefore, the thick portion imparts the required mechanical strength to that part while allowing the rest of the spring, which has a relatively small cross-sectional area, to decrease the spring constant of the spiral spring.
(4) In the power transmission mechanism according to the present invention, the cross-section of each arm of the spiral spring is preferably rectangular. For a given spring constant, the cross-sectional area of a spring having rectangular cross-section is smaller than that of a spring having circular cross-section. Therefore, a spring having rectangular cross-section has a smaller size and weighs less.
(5) In the power transmission mechanism according to the present invention, the spiral spring extends axially before being installed in the mechanism. When installed, the spiral spring is preferably axially compressed and connects the first rotating body with the second rotating body.
This structure requires only a relatively narrow space for the spiral spring. Thus, the space for the spring is readily available. Further, since the compressed spiral spring urges other members in the axial direction, the spiral spring may be used to urge an inner mechanism or an auxiliary mechanism of the driven apparatus. For example, in a subsequent description, a compressor, which serves as the driven apparatus, is coupled to an electromagnetic clutch, and a pulley (56) is driven by the drive source with a belt. An armature (58) of the clutch (55) engages with the pulley to rotate integrally with the pulley. The first rotating body includes the pulley (56) and the armature (58). In this case, the spiral spring urges the armature away from the pulley. When the electromagnetic clutch is deactivated, the spiral spring quickly disengages the armature from the pulley.
(6) In the power transmission mechanism according to the present invention, the spiral spring is preferably manufactured by pressing a metal plate. Pressing simultaneously forms the body of the spiral spring and parts that are coupled to the first rotating body and the second rotating body, which reduces the manufacturing cost of the spiral spring.